Easrwestoften,
During normal times we would be able to help more, however with Covid everything at the airport is being dictated by government policy/port health. This is changing regularly.
Make sure your daughter has the required Australian customs permission to leave the country by applying ahead of time on the website. As soon as she enters the terminal in Sydney she is going to have to produce that exemption number to prove she lives overseas.
She may have to settle any outstanding tax owning to get Australian government permission to leave. The process at the moment is very much thinking this is a one way forever deal. They limit the number of seats inbound to 50 per flight.
She will need to wear a mask to get on the CX flight.
The flight will be an education for her, you will see some people covered from head to toe in all different types of plastic from raincoats to grab wrap.
As explain up thread, she will need a full fare ticket SYD-LHR and check her luggage through to LHR to meet the HKG port health requirements.
You don’t want her being sent to a government camp for 14 days.
Make sure she reads the signs in the HKG terminal carefully and takes the transit passenger route.